Special amplifiers are used to amplify very weak signals. Quantum limited amplifiers (QLAs) are one such class of amplifiers. QLAs are finding uses in secure quantum communications, quantum cryptography, quantum computing, satellite based microwave communication systems, astrophysics research (such as dark matter searches or cosmic microwave background studies), and circuit-based quantum information processing. All of these applications involve sensing weak signals, for example, microwave signals, and require signal amplification in the presence of noise sources that may degrade or even destroy the information.
Preferably, a QLA would allow for operation over a wide frequency range, exhibit high gain, have a large dynamic range, and be compact and robust. However, it is difficult to achieve these characteristics simultaneously. Existing QLAs exhibit limited bandwidth, possess limited dynamic range, and require additional bulky microwave components that put physical design constraints on the above applications. For virtually all of the commercial applications discussed above, the bandwidth and the dynamic range of current state-of-the-art QLAs are not acceptable. Current QLA designs do not facilitate scalability and require far too much space in dilution refrigerators where they are most commonly used and where space comes at a significant cost.
Improvements in quantum information processing capabilities require overcoming a significant limiting technical obstacle: the parallel low-noise readout of quantum bits. Current technology is limited to the measurement of only a few quantum bits at a time due to the narrow bandwidth of current QLAs and the physical dimension of each measurement channel. A wide bandwidth quantum limited amplifier is necessary for the multiplexed readout of many quantum bits simultaneously.
Most of the applications for QLAs involve sensing weak signals near the single-photon level and require signal amplification in the presence of noise sources that may degrade or even destroy the information. Current amplification system technology suffers from several drawbacks making commercialization of the technology difficult.
First, operation of the amplifier at the quantum limit may involve cooling it in a dilution refrigerator to temperatures below 100 mK. To protect the amplifier and the device under test from noises in the system as well as thermal radiation from warmer stages in the dilution refrigerator, QLAs routinely include specialized cryogenic-grade circulators and isolators. Incorporation of circulators and isolators, which can be numerous, makes the system more complex.
Second, circulators and isolators are bulky components that greatly increase the size of the measurement system. The use of circulators and isolators also requires additional magnetic shielding and radio frequency (RF) cabling to room temperature, putting additional design constraints on the dilution refrigerator. The size, number, and complexity of the system with the required circulators, isolators, shielding, cabling, and refrigerator represent a significant cost. Ideally, a commercially viable QLA would obviate the need for a complex multi-component, bulky amplifier chain in these measurement setups.
Third, QLAs suffer from limited bandwidth, dynamic range, and signal gain. QLAs generally have internal noise levels smaller than the quantum mechanical fluctuations in the vacuum. This source of noise cannot be removed and represents the ultimate limit in the noise performance for any amplifier.
To address these limitations an ideal QLA would amplify in transmission mode, have a non-reciprocal gain, a large dynamic range, and no resonant structures to limit bandwidth.
U.S. Patent Application Publication No. 2012/0098594 discloses traveling wave parametric amplifier (TWPA) technologies based on superconducting NbTiN nanowires, which exploit the nonlinear kinetic inductance of the superconductor to parametrically amplify weak microwave signals. The main disadvantages of this amplifier are on-chip energy dissipation, operation above the quantum limit, elaborate microwave engineering requirements to inhibit generation of higher harmonics and suppress gain ripples, and difficult fabrication due to the high aspect ratio of a 1 μm×1 m long wire. The TWPA based on a superconducting nanowire has demonstrated a gain of 10 dB limited by the length and phase matching conditions of the signal, idler, and pump, which was insufficient to amplify the weak signal over the noise floor of a of the next amplifier in the measurement chain, typically a high electron mobility transistor amplifier (HEMT). Consequently, this type of amplifier is not suitable for most applications.